Do tubes have a break in period?

Yes they do.

They do, and I burn in new power tubes for 48 hours and then check the bias at 48 hours one more time because they seem to settle in at this point and the drift after that is minimal, at least for the KT77s/El34s I am using.

What I have found with both Valve Art and EH KT88s in my ASL Hurricanes is that they take a couple of days to stabilize bias current in a push pull amp. In that two day period, the bias fluctuated as the tubes burned in. This you need to watch closely for a few days before business as usual. I have noticed zero change in sound if kept biased within tolerance. Those who say they "warm up" are probably experiencing ear burn in rather than tube burn in - a phenomenon that makes us think it is getting better, but we are actually training ourselves or becoming acclimated to the new sound.

I had a V12R, Cary recommends 100 hours.

Though the internals of the 300B tube are larger than those of a lot of other tubes; the same components(cathodes/anodes/grids) & dynamics are present in all: (http://www.stereophile.com/content/search-perfect-300b-tube-page-3) (http://www.stereophile.com/content/search-perfect-300b-tube-tale-burn) Read the follow-up of the article as well, for further notes on burn-in(especially paragraph three).